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2.5. Side Chain Liquid Crystalline Polymers

Polymers with mesogens attached as side chains can exhibit liquid crystalline properties.

The extent to which the mesophase can develop in these systems is influenced by the flexibility of the backbone chain and whether the mesogen is attached directly to the chain or is pushed further away by the insertion of a flexible spacing unit.

The flexibility of the polymer chain can affect both the Tg and the temperature of the liquid crystal to isotropic phase transition Ti.

Polymer chain flexibility decreases in the order:Methacrylate > acrylate > siloxane.

So do the temperatures of the transition.

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The thermal range of the mesophase (T) is greatest when the chain is most flexible and its conformational changes largely do not interface with, or disrupt, the anisotropic alignment of the mesogens in the liquid crystalline phase.

When longer spacer units are introduced the observed effects are that the Tg of the polymer is usually lowered by internal plasticization.

As the spacer length is increased there is an enhancement of the ordering and nematic phase gives way to a smectic phase.

Ordering effects can also be encouraged by lengthening the alkyl tail unit.

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Long alkyl side chains tend to order and eventually to crystallize when long enough. This is also imposed on the liquid crystal state.Order parameter: where is the angle of mean deviation of the molecular axes of the mesogens with respect to the director. (S = 1 means perfect parallel orientation.)

The value of S for side chain liquid crystal polymers is approximately 75% of that obtained for the corresponding low molar mass mesogens and decreases with rising temperature as thermal agitation disturbs the orientation of the mesogenic groups.

The order state of the mesophase in the side chain liquid crystalline polymers is readily frozen into the glassy state if the temperatures dropped rapidly below the Tg and the value of S remains unchanged.

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cos(23

S 2

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This phenomenon offers unique applications in optoelectronic and information storage.

Liquid crystal polymers can be oriented by interaction with the surfaces of the measuring cell of glass plates.

This can give two extreme cases of rather (a) homogeneous or (b) homeotropic alignment where the long axes of the mesogens are respectively parallel to or perpendicular to the cell surface.

When viewed vertically to the glass plates through crossed polarizers, the systems are opaque or transparent respectively.

Where the relaxation time for the Frederisks transition to take place in low molar mass mesogens is of the order of seconds, viscosity effects in polymer systems can push this up several orders of magnitude larger.

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This may make the use of polymeric liquid crystals less attractive in rapid response display devices but the additional stability can be advantageous in other applications.

A special form of the nematic phase (N*), originally called the “cholesteric mesophase”, can often be detected in mesogenic systems containing a chiral center.A helical twist with a pitch p to the phase with a very high optical activity and the ability of selectively reflecting circularly polarized light of a specific wavelength R, when irradiated by normal light:

R = npwhere n is the average refractive index of the liquid crystalline phase.

2.6. Chiral Nematic Liquid Crystalline Polymers

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The synthesis of polymers capable of entering into a chiral nematic phase can be achieved by either copolymerizing the choesterol-containing monomers with other potential mesogenic monomer, or by synthesizing mesogens with a chiral unit in the tail moiety.

As the content of the chiral monomer (cholesterol) is varied in the copolymer, the wavelength of the reflected light changes, for an average temperature region between Tg and Ti.

The increase in percentage of cholesterol causes the helix twist to become tighter, i.e. p becomes smaller, so R also moves to shorter wavelengths.

When temperature is raised, the helix tends to unwind with a consequential increase in R.

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Cholesteric Phase (fan-like texture)

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Locking the chiral nematic phase into the glassy state by rapid supercooling to temperature below Tg leads to a preservation of the structure and reflected color, thereby lending to the formation of stable, light-fast, monochromatic films when suitable systems are used.

2.7. Miscellaneous Structures

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Figure 2. Twisted nematic device geometry. The polarizer and analyzer, which are arranged parallel to the director orientation at their adjacent glass plates, are oriented at 90 degrees to each other.

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Electrical Anisotropy: Dielectric Constant

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Multi-Domain Alignment

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Twisted Nematic Liquid Crystal Displays:Normally White (e-mode)

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Figure 1: (above) The LCI has created a 200 dpi cholesteric liquid crystal display that could be used in future portable reading devices. Figure 2: This 4" by 4", 80 dpi bistable reflective cholesteric display utilizes flexible polyester substrates.

Ferroelectric Liquid Crystal Display